Whistlers, Tweeks, etc.
If you've never heard a "whistler" or a "tweek" then you are in for a surprise. These are natural radio emissions usually triggered by lightning strikes and which are audible at very low radio frequencies. Equipment to receive these emissions is simple - just a sensitive audio amplifier, ideally with some rejection of mains hum and frequencies above about 10kHz, and a loop antenna.
Natural radio sounds can be quite unreal with whistlers sounding like a long descending pure or raspy whistle and tweeks sounding like the "ching" one gets when someone walks on a gravel path. Other emissions, called "chorus" sound like birds in a dawn chorus. The definitive work on the subject is "Whistlers and Related Ionospheric Phenomena" by R.A. Helliwell published by the Stanford University Press in the mid 1960s. The atlas section at the rear of the book contains some remarkable spectrogram pictures of VLF emissions. You can hear some of these at the links below.
Some years ago I recall hearing a whistler recording made in the atmosphere of Jupiter by a space probe - proof that lightning occurs in the Jovian atmosphere too.Natural Radio
VLF natural emissions page (Stanford University)